The present invention relates to terrestrial sensor devices intended to be placed on satellites or spacecraft and to aim at the Earth. These devices find a particularly important application in satellite attitude control systems, intended to align the satellite in its entirety, or elements which it carries, with the Earth or a determined region of the Earth. However, they are also usable for other purposes, such as the detection of large-scale fires on Earth.
In an important application, a function of the terrestrial sensor device is to measure position and the orientation of the satellite relative to the centre of the terrestrial disk. It provides information allowing the system to determine the misalignment angles in roll (that is to say along the North-South axis) and in pitch (along the East-West axis) relative to the centre of the Earth.
Currently, terrestrial sensor devices use individual elements, such as bolometers, placed at the focal plane of a lens and they include scanning means making it possible to determine the positions of the space/Earth and Earth/space transitions at a plurality of regions on the terrestrial disk. Most of the presently used devices scan, simultaneously or alternately, bands on either side of the equatorial plane. FIG. 1 shows a possible position of such bands 10a, 10b, 10c and 10d relative to the image 12 of the terrestrial disk, when the satellite is in its intented orientation. Only three of the four transitions a, b, c and d are necessary. The fourth transition provides redundancy and makes it possible to choose the most advantageous set of three bands when the solar position is such that the sun interferes with one of the scans.
One major drawback of these known devices is the requirement for scanning means having a mechanism with moving parts. Such a mechanism is complex and its reliability is limited because it must operate under the extreme conditions encountered in space. Given that detection takes place on single light sensitive elements, the integration time for the infrared light originating from a given point represents only a very small fraction of the measuring cycle.